In general, various types of display devices are used for computer and video systems including, for example, LCD (liquid crystal display) devices and LED (light emitting diode) display devices. A typical display device comprises a number of display elements or “pixels” arranged in rows and columns to form a matrix on a glass substrate. Active matrix backplanes, such as those used for driving OLED displays, typically include thin-film transistor (TFT) devices in the pixel circuitry, which operate as switching and driving components. An OLED generates light in response to current flow through an organic compound which is fluorescent or phosphorescent and excited by electron-hole recombination. Some known active-type OLED configurations incorporate two, three and four TFTs per pixel (2-TFT, 3-TFT, 4-TFT). A TFT-based OLED uses a TFT to control the amount of current flowing through the OLED based on data signals corresponding to a displayed image, which are received by the TFT. There are various disadvantages to active TFT-based OLED displays.
For example, the cost of the TFT backplane is a significant portion of the total display including the frontplane and packaging. Indeed, TFT backplanes are typically formed of low temperature poly silicon TFTs that are capable of delivering a large current and therefore, yielding a bright display. However, the poly silicon TFT fabrication process is expensive and complex as it requires many (e.g., nine) photoengraving process (PEP) steps to manufacture the TFTs. Moreover, the operation of the TFTs that drive an OLED can change over time, resulting in lack of uniformity of the current used to drive the OLED. For example, the threshold voltages of TFTs can vary over time due to electrical stress that is induced when driving OLED devices, as well as other factors or conditions that can temporarily or permanently change the threshold voltages of the TFTs. Since an OLED is a current-driven element in which the luminance depends on the amount of current flowing through the OLED, if the driving TFTs do not supply a uniform current, or if the driving current changes with time, the resultant image generated by the OLED display will degrade. For example, an increase in the threshold voltage of a driving TFT causes less current to pass through the OLED, thereby decreasing the brightness of the OLED.